In video coder/decoder systems, a video coder may code a source video sequence into a coded representation that has a smaller bit rate than does the source video and, thereby may achieve data compression. The video coder may code processed video data according to any of a variety of different coding techniques to achieve compression. One common technique for data compression uses predictive coding techniques (e.g., temporal/motion predictive coding). For example, some frames in a video stream may be coded independently (I-frames) and some other frames (e.g., P-frames or B-frames) may be coded using other frames as reference frames. P-frames may be coded with reference to a single previously coded frame (called, a “reference frame”) and B-frames may be coded with reference to a pair of previously-coded reference frames, typically a reference frame that occurs prior to the B-frame in display order and another reference frame that occurs subsequently to the B-frame in display order. The resulting compressed sequence (bit stream) may be transmitted to a decoder via a channel. To recover the video data, the bit stream may be decompressed at the decoder by inverting the coding processes performed by the coder, yielding a recovered video sequence.
To achieve high compression, the video coding processes typically are “lossy;” they permit a video decoder to recover a video sequence that is a replica of the source video sequence but has some errors. Thus, video coding systems often produce images with various types of coding artifacts including loss of detail, blockiness, ringing and banding. Such artifacts generally are more noticeable in still image content than in image content that exhibits a high degree of motion. Designers of video coding systems endeavor to provide coding systems that maintain high quality at appropriate bitrates and, therefore, avoid such display artifacts.